qemu.tests.cfg: Don't start vm in usb_multi_disk.max_disk test

The virt-test framework checks both 'start_vm' and 'restart_vm'
parameters before starting a VM. So if we don't want to start
VM by framework, we should set both of these parameters to 'no'.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
virt.virt_vm: Print usb device map when raising exception

The VMUSBControllerPortFullError exception doesn't provide
enough message to point out where has the problem.

This patch prints the usb device map when the exception is
raised, so that we can see which device caused the exception.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
virt.qemu_vm: Fix usb port assignment problem

If there are more than one usb controllers on VM,
the usb port assignment code would assign one port
on every controller. This patch tries to fix this
problem.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
qemu.tests.cfg: Force create images for usb testing

If we run the usb_multi_disk.max_disk several times,
this test would fail because the images are formatted
and no usable free space.

This patch makes these images created force everytime,
to avoid this problem.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
qemu.tests.usb: Wait for some time when getting usb disk name

This `usb_storage` script would fail when getting
usb disk name in guest, since the guest doesn't
fully boot up.
Thus it needs some more time to wait for guest
initiate usb disk after boot.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>